Lecture 14

Question 1

Function of the cytoskeleton?

Answer 1

-Structure/support
-Shape, cell migration, division
-Organization/transport

Question 2

Where is actin?

Answer 2

Right under the PM in the cell cortex

Question 3

Actin fucntion?

Answer 3

-Makes it so the cell does not deform
-Important for cell migration

Question 4

Function of microtubules?

Answer 4

-Important for transport
-Cell division (mitotic spindle)

Question 5

Function of intermediate filaments?

Answer 5

-not as important as actin/microtubules
-Important for mechanical strength

Question 6

G-Actin?

Answer 6

One actin subunit

Question 7

Actin structure?

Answer 7

-Made up of globular actin monomers
-Monomers have polarity

Question 8

How are actin filaments made?

Answer 8

-Monomers assemble in head to tail fashion (plus end to minus end)
-New monomers can be added to either end but are added more rapidly to the plus end.

Question 9

ATP-binding cleft of G-actin?

Answer 9

Near the minus end of actin

Question 10

What is nucleation?

Answer 10

When two monomers bump into one another the interaction is unstable(likely to fall apart) but if another monomer comes in prior to them falling apart they form a stable trimer that will be able to add more monomers before falling apart

Question 11

What controls the rate at which monomers are added to the F-actin?

Answer 11

The concentration of monomers nearby
High concentration = faster rate of addition

Question 12

Critical concentration?

Answer 12

When the rate of addition of monomers subunits is equal to the rate of disassembly of subunits

Question 13

Lag phase of actin?

Answer 13

-Monomers floating around and eventually three trimerize to form nucleus

Question 14

Growth Phase?

Answer 14

Once nucleation has occured subunits are rapidly added to the filament
-G-actin concentration decreases

Question 15

Equilibrium Phase?

Answer 15

-Rate of addition of new subunit decrease to match the rate of subunit dissociation
G-actin is at its critical concentration

Question 16

T/F: Actin can bind ATP/ADP

Answer 16

True

Question 17

G-actin is usually bound to which nucleotide?

Answer 17

ATP

Question 18

What happens when G-actin boudn to ATP is incorporated into F-actin?

Answer 18

1. G-actin ATP has high affinity for F-actin so it will bind
2. Once bound ATP hydrolysis will start to slowly occur
3. ADP bound actin now longer has affinity for F-actin and if it is at the end of the filament the actin will fall off

Question 19

Hydrolysis of ATP at the end of a filament vs middle ?

Answer 19

End : the monomer is likely to fall off
Middle: the monomer is less likely to fall off

Question 20

How does the ATP cap form?

Answer 20

If an ATP bound monomer binds to the F-actin before the monomer already in the F-actin hydrolyzes ATP
-New ATP bound monomers are added faster than the rate of hydrolysis

Question 21

Where does the ATP cap mainly occur?

Answer 21

Plus end

Question 22

Function of the ATP cap?

Answer 22

Favours growth of the actin filament

Question 23

Minus end of actin is typically?

Answer 23

All ADP actin

Question 24

Treadmilling?

Answer 24

When growth occurs at the plus end and loss occurs at the minus end
-Occurs at equilibrium where the filament stays the same length but the actins are being cycled through the filament

Question 25

Actin Nucleating Proteins?

Answer 25

Arp2/3 Formins

Question 26

Monomer binding proteins

Answer 26

-Profillin -Thymosin

Question 27

How does Arp2/3 work?

Answer 27

NPF binds Arp2/3 and snaps them together which forms a structure that resembles the plus end of actin. Now minus ends of the monomer add and start growing the actin filament

Question 28

When is Arp2/3 used?

Answer 28

When cell needs actin(cell crawling) -Creates branches off already formed filaments

Question 29

How do cells regulate Arp2/3?

Answer 29

Control the localization of NPF ex. For cell migration NPF will be near the front of the cell

Question 30

How do formins work?

Answer 30

-Sit at the plus end of filaments that have already started to form -Have N-terminal tails that grab nearby monomers by binding profiling -Accelerates growth of existing filament

Question 31

Where does profillin bind?

Answer 31

Plus end of monomers

Question 32

Purpose of Profillin?

Answer 32

-Binds plus end now growth can only occur at plus end of filaments -Allows other proteins(formins + NPF) to bind and recruit actin monomers -Speeds up growth of filaments

Question 33

What does Thymosin do?

Answer 33

Binds both the plus/minus end of actin monomers

Question 34

Function of Thymosin?

Answer 34

-Controls concentration of free actin(lowers it) -Monomers released by thymosin are then bound by profilin

Question 35

Where is tropomyosin found?

Answer 35

Muscle

Question 36

Purpose of Tropomyosin?

Answer 36

-Binds lengthwise along actin filaments - helps to stabalize and stiffen filaments

Question 37

What does CapZ do?

Answer 37

-Binds to the plus ends of filaments and prevents the addition and substraction of subunits -Minus end can still gain/lose subunits -Lowers the overall rate of filament assembly/diassembly

Question 38

Crosslinking proteins?

Answer 38

-Fimbrin - Alpha-actinin -Filamin

Question 39

Bundled networks vs Gel-like networks?

Answer 39

Bundled: whole of bunch of filaments next to each other that are parallel and organized Gel-like: contain filaments held at large angles from one another(disorganized, less uniform)

Question 40

Structure of fimbrin?

Answer 40

-Monomer -Two actin binding domains -Short molecule

Question 41

What does fimbrin do?

Answer 41

-Cross links actin into tight linked bundles -No flexibility -Important for filopodia

Question 42

Structure of alpha-actinin?

Answer 42

-Dimer -Two actin binding sites

Question 43

What does alpha-actinin do ?

Answer 43

-Filaments are held further apart and in parallel

Question 44

What are the proteins responsible for bundle networks?

Answer 44

1. Fimbrin 2. Alpha-actinin

Question 45

Protein responsible for gel-like network?

Answer 45

1. Filamin

Question 46

Structure of Filamin?

Answer 46

Monomer with two actin binding domain

Question 47

How does filamin work?

Answer 47

-Holds actin together in V-shape -Looser actin structure which allows more space for other proteins to bind to actin

Question 48

Gelsolin?

Answer 48

-Filament severing protein -Cuts the actin filament -Two binding domains (one for exposed actin and one for interior) -Stays bound to severed filaments to prevent reformation

Question 49

Coffilin

Answer 49

-Filament severing protein -Binds along side of F-actin -Twist filament tightly and causes it to fall apartH

Question 50

Who does coffilin bind to?

Answer 50

-Preferentially binds to ADP bound actin -Gets rid of old actin